Ultrastable halide perovskite CsPbBr3 photoanodes achieved with electrocatalytic glassy-carbon and boron-doped diamond sheets

Halide perovskites exhibit exceptional optoelectronic properties for photoelectrochemical production of solar fuels and chemicals but their instability in aqueous electrolytes hampers their application. Here we present ultrastable perovskite CsPbBr3-based photoanodes achieved with both multifunctional glassy carbon and boron-doped diamond sheets coated with Ni nanopyramids and NiFeOOH. These perovskite photoanodes achieve record operational stability in aqueous electrolytes, preserving 95% of their initial photocurrent density for 168 h of continuous operation with the glassy carbon sheets and 97% for 210 h with the boron-doped diamond sheets, due to the excellent mechanical and chemical stability of glassy carbon, boron-doped diamond, and nickel metal. Moreover, these photoanodes reach a low water-oxidation onset potential close to +0.4 VRHE and photocurrent densities close to 8 mA cm−2 at 1.23 VRHE, owing to the high conductivity of glassy carbon and boron-doped diamond and the catalytic activity of NiFeOOH. The applied catalytic, protective sheets employ only earth-abundant elements and straightforward fabrication methods, engineering a solution for the success of halide perovskites in stable photoelectrochemical cells.

This paper shows that protecting a CsPbBr3 solar cell with either glassy carbon or boron doped diamond can enhance its stability as a photoanode for water oxidation.This is a nice result and good stabilities have been found.The authors also show that a NiFeOOH catalysts is more effective when deposited on top of Ni pyramids.
The results are of interest and deserve to be published, but I question whether there are enough new results here for a high impact journal like NComms.The authors have previously published several papers using very similar systems and this is rather an incremental modification of the photoanodes reported in e.g Scalable All-Inorganic Halide Perovskite Photoanodes with >100 h Operational Stability Containing Earth-Abundant Materials, Advanced Materials, https://doi.org/10.1002/adma.202304350 Figure 7 makes it appear as if no perovskite photoanodes before the ones published here have stabilities over 70hrs (according to table S1).This is deeply disingenuous, two of the authors themselves have published stabilities of 110hrs for similar photoanodes (https://doi.org/10.1002/adma.202304350).Yang et al (Nature Communications volume 14, Article number: 5486 (2023)) also claim > 100hrs with UV filter.Fehr et al have shown >100hrs of unassisted water splitting using a tandem Nature Communications | (2023) 14:3797.
The band diagram in figure 2 suggests that the GC/Ni is sitting between -4.0 eV and -4.5 eV and the GC is at about -4.5 eV.Water oxidation is usually shown at energies between -5.0 eV and -6.0 eV (depending on the source), as drawn the GC/Ni should not be able to oxidise water?Please show both the forward and JV reverse scans for the solar cells in the SI.Can the authors also show the cells with short circuit currents close to 8 mA (e.g. in Figure S18 photocurrents of up to 8 mA are measured, but the JV curves show maximum 7 mA?) Figure 6 looks very strange -why was the light turned off at the different times chosen?It rather looks as if the data has been chopped just before the cells start to degrade?Otherwise, why were such random light off times selected?Why not measure all the cells to 210 hrs?
The authors reported perovskite-based photoanodes combined with glassy-carbon and boron-doped diamond sheets for durable PEC water oxidation.The photoanodes protected by glassy carbon sheets (boron doped diamond sheets) with low onset potential (+ 0.4 VRHE) retains 95% (97%) of initial photocurrent density after 100 h (210h) continuous operation, due to the excellent mechanical and chemical stability of glassy carbon, boron-doped diamond, and nickel metal.They proposed the potential of perovskite photoanodes as a promising method of durable solar fuel conversion.The manuscript is well written and clear.It can be interest of broad readership of Nature Communications by taking into account the below comments.
Comment 1: Device stability is a key factor limiting the cost and economic benefits of PEC systems to achieve industrial scale.What is the key factor that contributed to the enhanced stability?It appears that the perovskite also affects device stability.
Comment 2: In General, Spiro-OMeTAD is used as the hole transport layer in perovskite solar cell.Here a printing carbon paste was used instead of the HTL, what is the function of the printing carbon layer?Is it for reducing cost?Comment 3: In Fig. 6, the photoanode with BDD/NiFeOOH seems to be stable as well.What is the difference between the devices with BDD/NiFeOOH and BDD/Ni/NiFeOOH?The end of blue curve of photoanodes with BDD/NiFeOOH is covered by the red curve of GC/Ni/NiFeOOH, it could be revised to make it clear.
Comment 4: The stability of both devices with glassy carbon and boron doped diamond sheets are excellent.Could you evaluate the advantages of both protective sheets for large-scale preparation applications?
Comment 5: The adhesive layer was used to bond the carbon layer and the protective sheet and might affect hole transport.The result showed the conductivity was not changed too much.How to ensure the electrical conductivity? 1 Response to Reviewer Comments "Ultrastable halide perovskite CsPbBr 3 photoanodes achieved with electrocatalytic glassycarbon and boron-doped diamond sheets" by Zhu et al.
Manuscript submitted to Nature Communications (Research Article, We thank all the reviewers for their constructive feedback and for recognizing our manuscript's value and importance in terms of using glassy-carbon and boron-doped diamond sheets to achieve CsPbBr3 photoanodes with ultralong stability (>200 h at >95% preserved photocurrent).We have addressed all the concerns raised by the reviewers and have revised the manuscript based on them.We thank the reviewers for helping to refine and improve our manuscript with the help of their useful suggestions.We present below our detailed, point-by-point responses to the reviewers' comments.

Reviewer #1:
This paper shows that protecting a CsPbBr3 solar cell with either glassy carbon or boron doped diamond can enhance its stability as a photoanode for water oxidation.This is a nice result and good stabilities have been found.The authors also show that a NiFeOOH catalysts is more effective when deposited on top of Ni pyramids.

Response:
We are grateful to the reviewer for these insightful comments on our manuscript and for recognizing the outstanding stability of our CsPbBr3 photoanodes achieved with the protection of glassy-carbon and boron-doped diamond sheets.We have carefully considered the reviewer's points and revised the manuscript accordingly to address all concerns and questions.
1.The results are of interest and deserve to be published, but I question whether there are enough new results here for a high impact journal like NComms.The authors have previously published several papers using very similar systems and this is rather an incremental modification of the photoanodes reported in e.

Response:
Firstly, we thank the reviewer for their interest in our work.
We have only published two articles on the topic of halide perovskite photoanodes, one in Nat.Commun. in 2019 and another one in Adv.Mater. in 2023.The Nat. Commun.2019 paper covered the discovery of mesoporous carbon layers and graphite sheets to protect and create a halide perovskite photoanode.The Adv. Mater.2023 paper followed up the previous work and was focused on the 3D crystal phase engineering of CsPbBr3 for low-temperature photoanodes, the use of NiFeOOH as electrocatalyst, the scalability of the approaches, and the enhancement of photocurrents, however all of this was done by using the same graphite sheet protection as in the Nat.Commun.2019 paper.
Our current manuscript submitted covers different novel topics.The manuscript presents for the first time the use of glassy carbon and boron-doped diamond instead of graphite sheets for the protection of a halide perovskite, CsPbBr3, and the drastic enhancement of stability.These protective, catalytic sheets have never been used in combination with any semiconductor before in photoelectrodes, to the best of our knowledge.Moreover, we show the addition of novel Ni nanopyramids and NiFeOOH on these glassy carbon and boron-doped diamond sheets for further improvement of stability and enhancement of catalytic activity.The glassy carbon, the doped-boron diamond, and the Ni nanopyramids were not used in any of our previous works (or elsewhere) and so they are all novel aspects of our current manuscript.Furthermore, we also demonstrate a straightforward deposition of an adhesive layer on the backside of the glassy carbon and boron-doped diamond sheets, to attach the sheets to the photo-absorber structure.
The glassy carbon and boron-doped diamond sheets with Ni/NiFeOOH allow orders of magnitude longer water oxidation stability compared to the graphite sheets used in our two previous papers.Figure 6 of the manuscript (see also below its old version) compares their performance.Devices with a single graphite sheet never exceed 80 h stability (devices fully degrade, unless the graphite sheet is replaced), and by 60 h they already lose 20% of their initial photocurrent density.Our novel boron-doped diamond sample only loses 3% of its initial photocurrent density upon 210 h of continuous operation (i.e., it does not degrade and could be operated even longer).The glassy carbon one only loses 5% of its initial photocurrent density upon 168 h of continuous operation (also without any significant degradation).Importantly, the experiments in Figure 6 were simply ended by switching off the light.The excellent chemical and mechanical properties of glassy carbon and boron-doped diamond also avoids fluctuations of photocurrent, which is observed in graphite sheets due to their very porous, soft graphite layers that keep swelling during the photoelectrochemical operation and necessitates their replacement after 70 h of operation.Please see Figure R1 below, comparing photocurrents without normalization.One should note that a stable performance is needed in photoelectrodes and photoelectrochemical cells, since fluctuations would affect the selectivity in more complex reactions, such as glycerol oxidation to different products.

Old
Glassy carbon and boron-doped diamond sheets are non-porous and much more chemically and mechanically stable than graphite.Therefore, they withstand the harsh anodic condition in photoanodes and the damaging nucleation and growth of bubbles.In the new version of the manuscript, we have added further explanations and characterizations to address the Reviewers' comments which further emphasize the novelty and significance of our work (details in following responses).
We have also updated Figure 7 to make it more understandable, by readjusting the Y axis range to 0-1 and adding linear extrapolations in the background for visual aid, as shown in the following Figure Fig. 7 Comparison of normalized preserved photocurrent among PEC devices including halide perovskite photoanodes for solar-driven OER (AM1.5G, 1 sun) 23-31 .Further details in the Supplementary Table 1.Linear extrapolations in the background are added for visual aid.
2. Figure 7 makes it appear as if no perovskite photoanodes before the ones published here have stabilities over 70hrs (according to table S1).This is deeply disingenuous, two of the authors

Response:
We thank the Reviewer for their feedback on Figure 7. First, we would like to emphasize that a point in Figure 7 at for example 70 h and 83 % preserved photocurrent does not mean that this device could not last more than 70 h.To avoid this misunderstanding, we have added linear extrapolations in the background to all the points for visual aid (see Figure 7 in previous comment's response).We believe this visual aid makes the Figure clearer and puts more emphasis on the trends and not on the time the result was recorded.
Comparing photoelectrochemical results achieved in different labs is always difficult, and our last intention is to make a disingenuous Figure .When we compared the stability of our photoanodes with other works in Figure 7, we represented comparable measurements from different articles and included the details in Table S1 for transparency and frankness.The compared articles all report "photoelectrochemical devices with halide perovskite photoanodes for solar-driven OER", as literally explained in the Figure 7 caption.We excluded unrelated perovskite articles, such as those using a halide perovskite photocathode and a BiVO4 photoanode, since the focus of our work and Figure is on photoanodes and their harsher oxidative conditions.We confirm that Figure 7 includes the works cited by the reviewer, i.e., our previous two works (ref. 23 & 31), the work of Yang et al. (ref. 30), and the work of Fehr et al. (ref. 29).
We have just noticed that although Figure 7 and Table S1 report on the same articles, the reference numbers were not the same since the Supplementary Information reference list was restarting the numbering from 1.To avoid any confusion, now we have used the same referencing numbers in Manuscript and Supplementary Information (i.e.,. Here below we specify how we have fairly compared our work with these articles cited by the reviewer: ), a photoanode protected with one and two graphite sheets lasted around 40 and 70 h without replacement, respectively.We believe it is fairer to compare the stability of our glassy carbon and boron-doped diamond devices with graphite sheet devices without graphite sheet replacement halfway the experiment, since this is not done in the glassy carbon or boron-doped diamond devices reported in the submitted manuscript.This is explained in Table S1 note & that said "Without graphite sheet replacement".7 caption (note the words "for solar-driven OER").Therefore, we reported in Figure 7 and Table S1 that Yang et al. device reached 43% at 60 h time.We agree that Yang et al. achieves better stabilities using a UV filter and a "12 h dark recovery" at time 72 h, as shown in Figure R3, but these conditions are not comparable to our work.Including these results in Figure 7 comparison would be misleading to readers.Anyway, even with the 12 h recovery and UVfilter, Yang et al. just kept 57% of preserved photocurrent at 132 h, below our 95% and 97% of preserved photocurrent at 168 and 210 h with glassy carbon and boron-doped diamond, respectively.shows that a tandem made of halide perovskite photocathode and photoanode preserves 40% of the initial photocurrent density after around 16 h of operation.Since this is a tandem device, we included the note $ in the Supplementary Table 1 explaining details such as that the photocathode side alone kept 90% photocurrent after 60 h operation, so readers can infer that most degradation comes from the photoanode side (as expected due to its harsher oxidative conditions).Moreover, the authors also explained that "…The large variation in stability is a result of inter sample variability and changes in processing conditions, particularly relative humidity in atmosphere and exposure time, affecting the photoanode devices." in Supplementary Fig. 12  29 also showed >100 h stability.We are aware of those results.Such stability was reported in their Figure 3a and Supplementary Fig. 12 (ref. 29).The >100 h stability is for perovskite/silicon tandem photoelectrodes where the last layer in contact with water is not a halide perovskite layer but a silicon solar cell.Protecting silicon is not comparable to protecting a halide perovskite layer.Silicon is much easier to protect than halide perovskites because it is more chemically stable, more thermally stable, and flatter.For example, in Adv.Sust.Syst.2023, 7, 2300022 they use subsequent 15 min sonication in acetone, ethanol, and water to clean unprotected silicon photoanodes, something unthinkable for an unprotected halide perovskite.That silicon tandem structure is very different from our devices, so we believe it should not be included in Figure 7 and Table S1.In view of this comment, we have emphasized further the comparable conditions in the caption of Figure 7 and in Table S1, as follows: Fig. 7 Comparison of normalized preserved photocurrent among PEC devices including halide perovskite photoanodes for solar-driven OER (AM1.5G, 1 sun) 23-31 .Further details in the Supplementary Table 1.Linear extrapolations in the background are added for visual aid.

Supplementary Table 1 Comparison of reported photoelectrochemical devices including halide perovskite photoanodes for solar-driven OER (AM1.5G, 1 sun)
We have also added further explanation to the note $ on Supplementary Table 1  $ … The authors also report perovskite/silicon tandem photoanodes protected with GS/IrO x that achieve longer stabilities, but the last device layer on the aqueous electrolyte side is silicon, not a halide perovskite. 2 suggests that the GC/Ni is sitting between -4.0 eV and -4.5 eV and the GC is at about -4.5 eV.Water oxidation is usually shown at energies between -5.0 eV and -6.0 eV (depending on the source), as drawn the GC/Ni should not be able to oxidise water?

Response:
At 1 M NaOH (pH 14) the water oxidation is favored, with an oxidation potential located at -4.9 eV vs. vacuum (-4.9 = -4.5 + 0.83 -1.23 eV).The photoinduced holes from CsPbBr3 come from a deeper valence band edge at -5.7 eV vs. vacuum.Carbon, glassy carbon and boron-doped diamond behave as metals conducting the holes, so photoinduced holes transferred from the deeper CsPbBr3 valence band edge, especially when external bias is also applied will be able to oxidize water (i.e.hydroxyls).This is mainly due to the alignment of the quasi-Fermi level of holes with the metal work functions (considering only small bending of quasi-Fermi level within the transport layers).The quasi-Fermi level splitting in perovskite (and organic) solar cells is clearly not determined by the difference between the work function of the two metal electrodes (Trends Chem., 2019, 1, 1, 49-62., J. Appl. Phys., 2003, 93, 3605-3614.),which is demonstrated by devices with above 1 V Voc with similar work function electrodes (ACS Appl. Mater. Interfaces 2019, 11, 49, 45796-45804).As a consequence, under illumination the Fermi level of the top electrode will shift deeper together with the quasi-Fermi level of the perovskite, allowing for the oxidation of water (at -4.9 eV).This is further assisted by the application of an external bias, which will effectively push the metal work function deeper compared to the water oxidation potential.
In view of this comment, we have added the following to page 9:

Page 9:
The low Eon is the result of the low OER overpotential of GC/Ni/NiFeOOH and high photovoltage provided by the CsPbBr3 photo-absorber device, in agreement with the 1.17 V of Voc measured as a solar cell using electrical contacts on the FTO and GC/Ni/NiFeOOH (Supplementary Fig. 8).The high photocurrent density is in good agreement with the uniform large grains structure, the deep valence band edge supplying photoinduced holes with strong driving force for OER (Fig. 2c), interfacial energetics favoring charge separation (Fig. 2c), and the high light absorptance of the perovskite layer (Supplementary Fig. 1b).
4. Please show both the forward and JV reverse scans for the solar cells in the SI.Can the authors also show the cells with short circuit currents close to 8 mA (e.g. in Figure S18 photocurrents of up to 8 mA are measured, but the JV curves show maximum 7 mA?)

Response:
We have prepared a new device and measured both the forward and reverse JV scans with either a GC or BDD sheet (new Supplementary Figure 7).The open-circuit voltage of this device was slightly increased due to operational improvements.Some hysteresis is observed in agreement with literature (J.Mater.Chem.A, 2018, 6, 18677-18686).The PV hysteresis is less relevant for continuous operational studies (as herein performed in photoelectrochemical experiments under one selected applied bias condition and not scanning).
We did reach close to 8 mA cm -2 with a few champion photoanodes, but, as shown in Fig. S16, the average photocurrent values of photoanodes in the box plots are around 6-7 mA cm -2 .We believe it is better to show representative/average samples in the manuscript.We always observe good agreement between PV short-circuit photocurrents and PEC photocurrent plateaus, provided the OER reaction is favored with a catalyst such as NiFeOOH. 5. Figure 6 looks very strangewhy was the light turned off at the different times chosen?It rather looks as if the data has been chopped just before the cells start to degrade?Otherwise, why were such random light off times selected?Why not measure all the cells to 210 hrs?

Response:
We would like to provide some context.Continuous operational stability experiments are very time consuming and continuously occupy the whole photoelectrochemical workstation, limiting us doing other experiments.Figure 6 alone took almost two months.Measuring all the cells for 210 h would take almost 3 months, while measurements up to 135 h already allow for a good comparison of the different photoanodes.The light was switched off when considerable degradation was observed or when practically no degradation was observed for days, as follows:  The measurement of the photoanode with just GC was stopped at 65 h because already at 20 h the electrolyte had turned slightly yellow, as explained in page 15 and shown in Supplementary Figure 14.
 The one with GS/NiFeOOH was stopped at 80 h because the device had already lost 90% of its initial photocurrent.
 The one with GC/NiFeOOH was stopped when photocurrent reached a linear steep decay of 5% h -1 for 1 day, which was clearly irreversible.
 The one with BDD/NiFeOOH was stopped when no changes had been observed for 4 days.
 The one with GC/Ni/NiFeOOH was stopped when its photocurrent density had been constant for 5 days.
 The one with BDD/Ni/NiFeOOH was stopped when its photocurrent density had been constant for 8 days.
Overall, we still agree that the most important part of previous Figure 6 is the first 135 h that allows for the stability comparison of the different photoanodes.Since the full length of stability results for BDD/Ni/NiFeOOH and GC/Ni/NiFeOOH are also shown in Figure 4 and Figure 5, we have decided to update Figure 6 to show the results only up to 135 h, the time at which we stopped the measurement of BDD/NiFeOOH.We have now also added further clarification and context to the caption for each sample, as follows:

Reviewer #2:
The authors reported perovskite-based photoanodes combined with glassy-carbon and boron-doped diamond sheets for durable PEC water oxidation.The photoanodes protected by glassy carbon sheets (boron doped diamond sheets) with low onset potential (+ 0.4 VRHE) retains 95% (97%) of initial photocurrent density after 100 h (210h) continuous operation, due to the excellent mechanical and chemical stability of glassy carbon, boron-doped diamond, and nickel metal.They proposed the potential of perovskite photoanodes as a promising method of durable solar fuel conversion.The manuscript is well written and clear.It can be interest of broad readership of Nature Communications by taking into account the below comments.

Response:
We thank the reviewer for appreciating the low onset potential and unprecedented operational water oxidation stabilities achieved by our perovskite photoelectrodes applying novel materials.We are grateful for the suggestions that helped to improve the manuscript.
1. Device stability is a key factor limiting the cost and economic benefits of PEC systems to achieve industrial scale.What is the key factor that contributed to the enhanced stability?It appears that the perovskite also affects device stability.

Response:
In our study, the key factor that contributed to the enhanced stability was the use of glassy carbon or boron-doped diamond functionalized with Ni nanopyramids and NiFeOOH.This conclusion was possible because we chose relatively stable all-inorganic CsPbBr3 halide perovskite instead of typically unstable hybrid organic-inorganic halide perovskites.The good temperature and humidity stability of CsPbBr3 perovskite solar cells is known (J.Am.Chem. Soc., 2016, 138, 49, 15829-15832;ACS Appl. Mater. Interfaces, 2020, 12, 32, 36092-36101;ACS Appl. Mater. Interfaces 2019, 11, 33, 29746-29752).We agree that the perovskite can also affect device stability, but this was avoided in our careful selection of materials for unambiguous results.We also chose stable transport layers such as SnO2 and mesoporous carbon layer and avoided air-unstable transport layers such as Spiro-OMeTAD. 29With this design, we could focus on the stability enhancement by the catalytic, protective sheets and unambiguously show the superiority of glassy carbon (GC/Ni/NiFeOOH) sheets and boron-doped diamond (BDD/Ni/NiFeOOH) sheets over the state-of-the-art protective sheets made of graphite and NiFeOOH.We further discuss the stability topic in our response to Reviewer 3 comment 8.
2. In General, Spiro-OMeTAD is used as the hole transport layer in perovskite solar cell.Here a printing carbon paste was used instead of the HTL, what is the function of the printing carbon layer?Is it for reducing cost?

Response:
As explained above, expensive Spiro-OMeTAD is known to degrade in air, 29 and we managed to achieve good photocurrents and photovoltage with inexpensive carbon paste as top electrode.The advantages of printed carbon layer on top of the perovskite are various.Yes, it reduces costs but also easily covers a rough perovskite layer, ensures good electrical contact with the protective sheets made of graphite, glassy carbon or boron-doped diamond, and is also a hydrophobic protective layer.If used alone, it will protect the perovskite for up to 2-3 h in aqueous electrolyte.
In a previous work (ref. 31), we prepared CsPbBr3 photoanodes with Spiro-OMeTAD HTL between the perovskite photoactive layer and the printed carbon; however, we found that the devices with HTL showed similar performance or underperformed the ones without HTL.This suggests that in our devices the main origin of non-radiative recombination losses is not at the perovskite/HTL interface, probably due to the charge transfer already being relatively efficient because of large interfacial bend bending at the SnO2/CsPbBr3 interface.Moreover, the shallow highest occupied molecular orbital (HOMO) of Spiro-OMeTAD (-5.0 eV) compared to the deep valence band edge of CsPbBr3 (-5.7 eV) might also limit the use of Siro-OMeTAD as HTL in CsPbBr3-based devices.Yang et al. in ref. 30 also made similar observations on the deep valence band edge of FAPbBr3.
3. In Fig. 6, the photoanode with BDD/NiFeOOH seems to be stable as well.What is the difference between the devices with BDD/NiFeOOH and BDD/Ni/NiFeOOH?The end of blue curve of photoanodes with BDD/NiFeOOH is covered by the red curve of GC/Ni/NiFeOOH, it could be revised to make it clear.

Response:
We agree that the photoanode with BDD/NiFeOOH was stable too.However, the onset potentials for water oxidation on BDD/NiFeOOH are worse than on BDD/Ni/NiFeOOH.The same difference is observed between GC/Ni/NiFeOOH and GC/NiFeOOH (as shown in Fig. 3c of the manuscript).Therefore, we still recommend using both Ni and NiFeOOH on GC and BDD.This was discussed in our manuscript Page 14 as "The photoanode with BDD/NiFeOOH exhibits stable photocurrent for most of the time; however, the photocurrent decreased in the first few hours, and the onset potential and actual current density are lower than that of the device with BDD/Ni/NiFeOOH sheet…").The Ni nanopyramid layer on BDD surface provides more surface area for the deposition of NiFeOOH catalyst, as shown in Supplementary Fig. 12.
We thank the reviewer for pointing out the blue curve being covered in the figure.We have updated Fig. 6 to make it clearer.Please see the revised figure below.4. The stability of both devices with glassy carbon and boron doped diamond sheets are excellent.Could you evaluate the advantages of both protective sheets for large-scale preparation applications?

Response:
We believe that the main advantages are their chemical and mechanical stability, which resulted in the excellent results obtained, as discussed/demonstrated in the manuscript.The mechanical properties of glassy carbon and boron-doped diamond (rigid like glass) also impart better mechanical properties to the final device, forming a sandwich with the glass support.Moreover, unlike graphite sheets, glassy carbon and boron-doped diamond sheets can be reused multiple times since only their surface is modified.Better chemical and mechanical properties, as well as reusability, will favor their application to large-scale devices.
We believe that large-scale preparation of photoelectrochemical devices will follow a modular approach, in which devices of a few centimeters (e.g.50-100 cm 2 ) will be repeated in series/grids and utilize concentrated sunlight.This approach minimizes mass transport and resistivity limitations in the glass/FTO support (Hankin et al., Adv. Energy Mater. 2021, 11, 2003286).Our design approach with glassy carbon and boron-doped diamond sheets coated with Ni/NiFeOOH will have no limitation in such large-scale preparation.There are already commercially available GC and BDD sheets of 100 cm 2 .
In view of this comment, we have added the following sentence to page 15: …The narrow standard deviation of photocurrents and Eon confirms the reproducibility of the device fabrication procedure and the robustness of the reused GC and BDD sheets, as well the good economic practicability of our electrocatalytic protective sheets for large-scale production.
5. The adhesive layer was used to bond the carbon layer and the protective sheet and might affect hole transport.The result showed the conductivity was not changed too much.How to ensure the electrical conductivity?

Response:
Roughness is key.The printed carbon is porous and rough.When the GC or BDD sheet is placed on top of the printed carbon, the glue can be squeezed into surface pockets and direct electrical contact can occur between the spiky printed carbon flakes and the GC or BDD sheet surface.This was represented in the Supplementary Figure 6 sketched diagram (here below for easier reference).Moreover, the dissolution of the adhesive in toluene (adhesive:toluene 1:3 vol.) ensured a thin and homogenous adhesive layer upon spin coating.Without dilution, non-homogenous layers and higher resistance values were achieved.However, too much dilution resulted in insufficient adhesion.A volumetric ratio of 1:3 was found optimal.
In view of this comment, we have rewritten a paragraph in page 9 and added some words and sentences to page 19 and Supplementary Figure 6 caption, as follows: Page 9: After successfully demonstrating the OER catalytic activity of the GC/Ni/NiFeOOH sheet, we focus on the PEC performance and stability of the CsPbBr3 photoanode protected with it (Fig. 4).The protective sheet was adhered and electrically contacted to the CsPbBr3 photo-absorber device by using a thin adhesive layer prepared by diluting a commercial adhesive in toluene and spin coating.To confirm the electrical contact across the interfaces, 2-electrode J-V scans of FTO/carbon and FTO/carbon/adhesive/GC (and later BDD) were measured, showing similar high slope values, that is, similar small resistances of only 3-4 W (Supplementary Fig. 6).The surface of printed carbon is rough, ensuring the presence of surface pockets for the adhesive and spikes for the direct electrical connection to the GC (and later BDD) sheet, as sketched in Supplementary Fig. 6.Therefore, no conductive fillers are needed in the adhesive layer provided there is enough roughness at the interface. 29Moreover, the GC (and BDD) sheets are very conductive, 45 (and 10) μΩ m, so the resistance across their 1 (and 0.8) mm thickness is negligible, only 450 (and 80) μΩ.

Supplementary Fig. 6 (a) Sketch of resistance measurement across FTO/mesoporous carbon (MC), FTO/MC/adhesive (AD)/GC, and FTO/MC/AD/BDD interfaces. The printed carbon layer is rough and porous, ensuring the presence of pockets for the adhesive and spikes for the electrical contact to the GC or BDD sheets.
Page 19 Experimental: …A commercial adhesive (RS PRO Spray Adhesive, RS) was diluted in toluene (volume ratio of 1 adhesive : 3 toluene) and spin-coated on the back GC side of the protection sheet at 2500 rpm for 15 s.The dilution with toluene ensured the formation of a homogenous thin adhesive film upon spin coating.A volume ratio of 1:3 (adhesive:toluene) was found optimal to avoid electrical resistance while maintaining adhesion.The… 6.In Figure 5c, photograph of the photoanode under operation is not clear to see the bubbles, it should be revised.In Supplementary Fig. 8c, the "Ni metal" is outside of the figure, it should be revised.

Response:
Thanks for the useful comments.We have modified both figures to the following clearer versions: Supplementary Fig. 10 (a) LSV under 1 sun (solid lines) or in dark (dashed line).(b) Applied bias photon-to-current efficiency (ABPE) of photoanode protected with GC/Ni/NiFeOOH (the data is calculated from the values of Fig. 4a).(c-e) XPS spectra of GC/Ni/NiFeOOH protected photoanode before and after 168 h photoanode stability measurement (presented in Fig. 3d).The peak increase at 0.45 VRHE was caused by the surface activation (i.e., oxidation) of the Ni nanopyramids during the stability test.

Reviewer #3:
In this manuscript, Zhu et al. demonstrated the effectiveness of the glassy carbon and boron-doped diamond sheets coated with Ni nanopyramids and NiFeOOH in protecting the metal halide perovskite photoanode in photoelectrochemical water splitting cells.The fabricated photoelectrochemical cells exhibited a record operational lifetime in the field.Nevertheless, the performance parameters of the device require more comprehensive assessment.From this reviewer's perspective, the work involved significant amounts of engineering works, but scientific findings were limited, thus limiting the novelty and significance of this work.Further measurement is needed to support the conclusion.At this stage of performance evaluation and scientific discussion provide in the manuscript, this reviewer cannot recommend this manuscript to be published in nature communication.To be considered for publication, the authors can consider revise the manuscript regarding to following concerns:

Response:
We thank the reviewer for appreciating the record operational lifetimes and significant amount of engineering work.We, however, believe the article also shows scientific findings, novelty, and significance, since this is the first work to show the use of glassy carbon and boron-doped diamond sheets to protect a water-unstable semiconductor and to achieve record operational stabilities with a halide perovskite photoanode.We thank the Reviewer for their suggestions that helped to improve the manuscript and its fitting in Nature Communications.
1.A good protective layer should be able to protects the photo-absorber and transport layers from the corrosive electrolytes without compromising the transportation of photogenerated charges.Although the fabricated protective sheets can provide excellent protection, the author should provide more assessment in term of performance compromise.

Response:
We had included plenty of assessment and characterization of the performance of our devices, including:  Energy diagrams of all the components (Figure 2c, 5b, S1c-d, S11, …)  J-V curves measured as a solar cell (Figure S8)  J-V curves measured as photoanodes for solar OER (Figure 2c, 4a, S10a, S13b, S16, S17…)  J-V of catalytic sheets for OER (Figure 3c, 3d, S5, S6…  Microscopies of the different components (Figure 2b, 3a, 5a, S2, S3, S4, S12, S15…)  Anodic operational stability studies (Figure 3d, 4d, 5c, 6…) And now, according to the Reviewer's comments, we have added more new measurements to support our conclusion, including:  J-V curves of devices without and with GC or BDD (new Figure S7)  Photoluminescence measurements of devices without and with GC or BDD (Figures S6c-d)  Applied bias photo-to-current efficiency of photoanodes (new Figures S10b and S13a)  PV stability of devices (Supplementary Figure S9)  AFM surface area of protective sheets (Figure S12) These characterizations and tests provide a clear picture of the performance of a halide perovskite photoanode protected with glassy carbon and boron-doped diamond sheets with Ni nanopyramids and NiFeOOH.The characterization is done with rigor and detail, and clear links and comparisons to literature put in context the scientific relevance and novelty of the results.Details are provided in the following responses to more specific comments.

Response:
We have calculated the applied bias photon-to-current efficiency (ABPE) of our photoanodes with GC/Ni/NiFeOOH and BDD/Ni/NiFeOOH and added the results to the new Supplementary Figures 10b and 13a.The maximum ABPEs of photoanodes with GC/Ni/NiFeOOH and BDD/Ni/NiFeOOH are 2.45% and 3.84%, respectively.Although higher values were previously achieved with hybrid organic-inorganic halide perovskites such as MAPI, their instability under light conditions of such perovskites would have created confusion if chosen for this study.CsPbBr3 has a relatively large bandgap of 2.3 eV, which results in lower values of ABPE; however, it was chosen for this work as a photoactive material due to its excellent stability.Readers would have struggled to distinguish the GC/Ni/NiFeOOH and BDD/Ni/NiFeOOH stability on a photoabsorber that is not stable.As explained to Reviewer 2, using relatively more stable CsPbBr3 allowed us presenting a clear comparison between our proposed GC and BDD sheets and state-of-the-art graphite/NiFeOOH sheets.Solar-to-hydrogen (STH) efficiency should be calculated in a two-electrode system, without any assistance of external applied bias.This would require the addition of a photocathode in a tandem configuration to generate enough photovoltage and photocurrent at operation point.Photocathodes (under reducing conditions) are typically more stable than photoanodes (under oxidation conditions).The STH conversion efficiency should take into account the entire PEC cell, including photoanode, photocathode, electrolyte, membrane, cables, etc.The development of photocathodes and an entire tandem cell is out of the scope of this manuscript that is focused on the stability of halide perovskite photoanodes under harsh oxidation conditions.
The performance of photoanodes in literature is typically compared in terms of JV curves (presented in Figure 2c, 4a, S10a, S13, S16, S17, …) and incident photon-to-current efficiencies (IPCE).The IPCE results were presented in the manuscript Figure 4b.
In view of this comment, we have added the ABPE figure (Supplementary Fig. 10b and 13a and the following text: Page 9: … The applied bias photon-to-current efficiency (ABPE) was calculated to have a maximum of 2.45% at +0.7 VRHE (Supplementary Fig. 10b).
Figure 7 and Supplementary Table 1.The VOC of this device was higher due to some operational improvements in the fabrication and PV measurement steps.It is clear that the addition of very conductive protective sheets had neglibible effect on their PSC performance, which we further confirmed by the PL measurements (next comment's response).The realtively low FF can be attributed to the perovskite layer, which we are planning to improve in further work.
In view of this comment, we have added more detailed discussion in the manuscript (Page 9) and the following Supplementary Fig. 7 and Supplementary Table 1:   Page 9: …Furthermore, the photovoltaic (PV) performances of a device without and with protective sheets were compared (Supplementary Figure 7).Practically the same performance was observed upon addition of the protective sheet, assigned to its high conductivity (resistivity of GC: 45 μΩ m, BDD: 10 μΩ m).Supplementary Fig. 7 ( 4. The author should provide measurement of the charge transfer between perovskite and the protective layers (e.g.photoluminescence), the conductivity measurement provided is not sufficient to confirm the efficient charge transfer.

Response:
According to the Reviewer's suggestion, we conducted photoluminescence (PL) measurements through the back of the samples (glass side) with all the transport layers without and with the protective sheets on top of the printed carbon layer.Due to the measurement configuration, the PL intensity is smaller than usual, and the PL spectra are less symmetric (new Supplementary Figure 6c-d).The results showed that there are no differences in PL intensity between the device without and with protective sheet on top of the printed carbon layer.Moreover, there is no PL shift.Therefore, there is negligible non-radiative recombination caused by the GC or BDD sheet addition on top of the carbon layer.The charge transfer between the perovskite and the protective layers is not affected.
In view of this comment, we have added more discussion in page 9 and 21 and supplementary Fig. 6: Page 9: …To further characterize the devices, we conducted photoluminescence (PL) measurements through the back of a sample (glass side) with all the transport layers without and with a GC (and BDD) protective sheet on top of the printed carbon layer.Due to the measurement configuration, the PL intensity is smaller than usual, and the PL spectra are less symmetric (Supplementary Figure 6c-d).The results showed that there are no differences in PL intensity between the device without and with protective sheet on top of the printed carbon layer.Moreover, there is no PL shift.Therefore, there is negligible non-radiative recombination caused by the GC (or BDD) sheet addition, and the charge transfer between the perovskite and the protective layers is not affected.
Page 21 Characterization: An FLS1000 (Edinburgh Instruments) photoluminescence spectrometer was used to obtain steadystate photoluminescence (PL) spectra with a xenon arc lamp (450 W, ozone free).The excitation wavelength was set at 405 nm with a bandwidth of 8 nm.Supplementary Fig. 6 … (c-d) Photoluminescence (PL) spectra of the CsPbBr3 photoanodes without and with GC and BDD sheets.This PL was measured from the back of the samples with all the transport layers, so the PL intensity is smaller than usual and the shape of PL is less symmetric.
5. Please provide more detail discussion on the origin of low onset potential achieved.Such as the effect of energy band alignment to the potential.

Response:
The low photoanode onset potential results from the low OER onset potential of the Ni/NiFeOOH catalyst layer and the large photovoltage provided by the CsPbBr3 photoabsorber device.
The Ni/NiFeOOH evolves oxygen with a low onset potential at 1.55 VRHE due to the good catalytic activity of NiFeOOH and the Ni nanopyramid structure that provides large surface area for both NiFeOOH deposition and posterior OER reactions (Fig. 3c and Supplementary Fig. 12).Moreover, Supplementary Fig. 5 shows the Tafel plot of different protective sheets.The Tafel slopes of GC and BDD were reduced with the addition of Ni nanopyramids and NiFeOOH, indicating a more responsive current and OER to the applied voltage upon Ni/NiFeOOH loading.
The suitable energy band structure promotes the electron-hole separation and charge transfer for the OER with the photoinduced holes (Fig. 2c).This was explained in pages 5, 7, 9 and 12 of the manuscript as follows:  Page 9: … The high photocurrent density is in good agreement with the uniform large grains structure and interfacial energetics favoring charge separation (Fig. 2) and the high light absorptance of the perovskite layer (Supplementary Fig. 1b).
 Page 12: …Therefore, the improved performance of BDD devices compared to those of GC is in part assigned to its increased surface area, which promotes charge transfer and oxygen evolution by providing more reaction sites and interface to the electrolyte.
We believe that all these explanations and photoelectrochemistry literature cited in the manuscript provide a clear picture on the origin of the low onset potential for solar OER with our photoanodes.And in view of this comment, we have added the following to page 9: Page 9: The low Eon is the result of the low OER overpotential of GC/Ni/NiFeOOH and high photovoltage provided by the CsPbBr3 photo-absorber device, in agreement with the 1.17 V of Voc measured as a solar cell using electrical contacts on the FTO and GC/Ni/NiFeOOH (Supplementary Fig. 8).The high photocurrent density is in good agreement with the uniform large grains structure, the deep valence band edge supplying photoinduced holes with strong driving force for OER (Fig. 2c), interfacial energetics favoring charge separation (Fig. 2c), and the high light absorptance of the perovskite layer (Supplementary Fig. 1b).
6. From the Supplementary Fig. 7, it seems that the fill factor of the PSC device was quite limited, is this due to the effect of the GC/Ni/NiFeOOH sheets?

Response:
The GC/Ni/NiFeOOH does not limit the PSC performance because GC is very conductive, so any limited fill factor is attributed to the quality of the perovskite layer.We refer to our response to comment 3, in which we explained that we prepared a new device and measured its JV without and with GC/Ni/NiFeOOH sheet (new Supplementary Figure 7a).The fill factors of the same device without and with GC/Ni/NiFeOOH sheets were 0.62 and 0.60, respectively, which is practically the same.Therefore, the GC sheet does not limit the device.
7. In figure 4c, why the experimental amount of O2 > the theoretical amount.I supposed it was a typo error.

Response:
Thanks for finding this typo error.We have now corrected the figure and updated it in the manuscript as follows: 8.The photoelectrochemical cells show a 5% decrease in the photocurrent after 168 hours.I wonder would this be due to the degradation of the catalytic sheet or the degradation of the perovskite absorber.
To clarify this, could the author provide a maximum power point tracking for the device under 1 sun conditions when working as a solar cell in ambient conditions.

Response:
Following the advice, we have measured the PV stability of a CsPbBr3 device with GC for 235 h.A maximum power point (MPP) measurement would constantly track MPP and adjust voltage, meaning a continuous changing of applied potential throughout the stability measurement.MPP testing condition would be different from PEC stability tests done at constant applied bias of +1.23 VRHE.Therefore, we have measured the PV stability at a 0.2 V to make it most comparable to the PEC stability measurements, which were measured at 0.2 V vs Ag/AgCl electrode (+1.23 VRHE at pH = 14) (new Figure S9).The device kept a stable photocurrent.The initial photocurrent density jph was 5.7 mA cm -2 and 89% and 88% of it was maintained after 168 and 210 h of operation, respectively.This jph decrease is slightly larger than what we observed in PEC measurements (11 vs. 5%), but practically the same if we consider the maximum jph (instead of the initial jph) achieved at 15 h time in Figure 4d (11 vs. 10%).The operational PV stability of CsPbBr3 is in agreement with literature (ACS Appl.Mater.Interfaces, 2020, 12, 32, 36092-36101;ACS Appl. Mater. Interfaces 2019, 11, 33, 29746-29752).
The PV stability results indicate that some decrease of performance can be assigned to the perovskite photoabsorber.We have also observed a small overpotential increase over time in the catalytic sheets (Figure 3d).Therefore, the small decrease in PEC activity can be assigned to both the perovskite absorber and the catalytic sheet.In both cases, the loss of performance is small, which makes them difficult to distinguish.
In view of this comment, we have added the following text and new Supplementary Fig. 9.
Manuscript page 8: …The Tafel plots of the various protective sheets were also calculated, confirming the superiority of GC/Ni/NiFeOOH with a Tafel slope of 101 mV dec -1 (Supplementary Fig. 5).The OER overpotential stability of the GC/Ni/NiFeOOH sheet for 20 h at 10 mA cm -2 is shown in Fig. 3d and confirmed to be stable, with only a small increase over time.
The photoanode was tested in a three-electrode setup at +1.23 VRHE under 1 sun illumination.The chronoamperometry of the device for OER measurement is illustrated in Fig. 4d.The photoanode shows excellent stability, demonstrated for 168 hours (8 days) with a final photocurrent density of 5.5 mA cm -2 (95 % of the initial performance).During the first 15 h, the photocurrent density slightly increases to 6.1 mA cm -2 probably due to activation of the Ni and NiFeOOH layers under OER conditions.From the 15 h on, the photocurrent is very stable, only decreasing 0.0037 mA cm -2 h -1 .The device measured as a solar cell shows similar small decay, confirming the good translation of stability from solar cell to PEC application (Supplementary Fig. 9).The PEC workstation lamp is turned off at the 168 hour for post-test characterization, but the device still works.The Ni nanopyramid structure of the protective sheet shows no change after the long 168 h stability test (inset SEM micrograph in Fig. 4d)… Supplementary Fig. 9 PV stability at 0.2 V voltage of a CsPbBr3 device covered in GC sheet under 1 sun illumination.Initial photocurrent density jsc: 5.7 mA cm -2 .9. Would the thickness of the protective sheet affect the stability and the compromise of device performance?Please provide details about the thickness of the protective sheet.

Response:
The thicknesses of used GC and BDD sheets were 1 mm and 0.8 mm, respectively, as explained in the Experimental.These thicknesses were convenient to safely work with the sheets (roughening with abrasive paper, cleaning, spin coater and Ni/NiFeOOH electrodeposition).In terms of stability, the addition of Ni/NiFeOOH layer protects the GC or BDD surface exposed to the harsh electrolyte for solar OER conditions as well as makes the sheet catalytically active.Provided the Ni/NiFeOOH is homogeneously deposited on the GC or BDD surface as presented in the manuscript, the sheet thickness will not affect stability.However, if no Ni/NiFeOOH is deposited, the GC will degrade with time under solar OER operation (while BDD is stable) as shown in page 14 and Supplementary Figs.
14 and 15.In that case, a thicker GC sheet will obviously offer longer stability, but it is not recommended because photocurrents will be poorer anyway due to the absence of Ni/NiFeOOH catalyst.
In terms of performance, glassy carbon and boron-doped diamond are very conductive.The specific resistances  of GC and BDD are 45 μΩ m and 10 μΩ m (data provided by the manufacturer), respectively, so the resistances across the opposite faces of 1 cm 2 GC and BDD sheets are just 0.00045 Ω and 0.00008 Ω.The measured total resistance across the FTO/carbon/adhesive/GC (or BDD) interfaces were 3-4 Ω per cm 2 , as shown in Supplementary Fig. 6 and explained in page 9. Therefore, the inner resistances of GC and BDD due to their thickness are irrelevant, and they will not affect the final performance.
In view of this comment, we have added the following to the manuscript:

Page 9:
After successfully demonstrating the OER catalytic activity of the GC/Ni/NiFeOOH sheet, we focus on the PEC performance and stability of the CsPbBr3 photoanode protected with it (Fig. 4).The protective sheet was adhered and electrically contacted to the CsPbBr3 photo-absorber device by using a thin adhesive layer prepared by diluting a commercial adhesive in toluene and spin coating.To confirm the electrical contact across the interfaces, 2-electrode J-V scans of FTO/carbon and FTO/carbon/adhesive/GC (and later BDD) were measured, showing similar high slope values, that is, similar small resistances of only 3-4  (Supplementary Fig. 6).The surface of printed carbon is rough, ensuring the presence of surface pockets for the adhesive and spikes for the direct electrical connection to the GC (and later BDD) sheet, as sketched in Supplementary Fig. 6.Therefore, no conductive fillers are needed in the adhesive layer provided there is enough roughness at the interface. 29Moreover, the GC (and BDD) sheets are very conductive, 45 (and 10) μΩ m, so the resistance across their 1 (and 0.8) mm thickness is negligible, only 450 (and 80) μΩ per cm 2 sheet area.…

Page 18:
Protective catalytic sheet fabrication Protective sheets consist of a substrate (GC sheet or BDD sheet), catalytic layers, and adhesive layer.Before fabricating each protective sheet, the front sides of the GC sheets (JM Material CO., Ltd., 10 mm × 10 mm × 1 mm, resistivity: 45 μΩ m)… For the devices with BDD sheets (CVD BDD sheet,Ningbo Gh Diamond Tool CO.,Ltd.,10 mm × 10 mm × 0.8 mm,resistivity: 10 μΩ m, one side polished),… 12.The active surface area of the electrode is 0.197 cm2.How about the active area (illuminated area) of the perovskite solar cells?

Response:
The illuminated area of the perovskite solar cell was the same as the reaction surface area (0.197 cm 2 ).We used circular masks on both sides to control the illuminated and wet areas and for calculating the photocurrent density.
In view of this comment, we have added more context in (Photo)electrochemical measurements section as follows: Page 20: …(circular mask with predefined diameter of 5 mm was used to control the active surface area 0.197 cm 2 ) against a Pt counter electrode and a KCl-saturated Ag/AgCl reference electrode.The illuminated area was also controlled to be the same as the active surface area by using a front circular mask.
13. Line 279, page 12 in the manuscript, the authors assigned the improved performance of BDD device compared to the GC is due to the higher surface roughness thus more reaction sites.Does that mean due to the increased surface area?Should the author provide a specific surface area measurement to support this point?Because increased roughness does not necessarily lead to increased surface area.

Response:
Yes, we meant due to increased surface area.It is well-known in electrocatalysis that a higher surface area of electrocatalyst favors catalysis, in this case for OER.We have calculated the surface area of the catalytic sheet front surface by AFM.Surface areas (Sa) of front side of GC and BDD were 1.017 m 2 and 1.084 m 2 per m 2 of projected area (m 2 m -2 ), respectively.Surface areas of GC/Ni/NiFeOOH and BDD /Ni/NiFeOOH were 1.096 m 2 m -2 and 1.561 m 2 m -2 , respectively.The surface area of BDD/Ni/NiFeOOH is therefore larger than that of GC/Ni/NiFeOOH sheet, which provides more reaction sites for OER, so higher PEC photocurrents.This is also evident in the AFM micrographs (Supplementary Fig. 12e-f.),there are more nanopyramids structures on the BDD surface for the deposition of NiFeOOH electrocatalyst.
In view of this comment, we have added more discussion in the manuscript and Supplementary Fig. 12 as follows: Page 12: …The surface area Sa values of the front side of GC before and after Ni and NiFeOOH deposition are 1.017 and 1.096 m 2 per m 2 of projected area (m 2 m -2 ), respectively, and those of BDD are 1.084 and 1.561 m 2 m -2 , respectively.Therefore, the improved performance of BDD devices compared to those of GC is in part assigned to its double surface roughnessincreased surface area, which promotes charge transfer and oxygen evolution by providing more reaction sites and interface to the electrolyte.
Anyway, our manuscript is not focused on scale-up or carbon layer sheet resistance, but on the presentation of GC and BDD protective sheets with Ni/NiFeOOH for photoanodes.In a previous article in Adv.Mater.2023 using graphite protective sheets (that are also very conductive), we already scaled the carbon-based PSC devices to 113 mm 2 .Going larger is not easier, and it is pure engineering and out of the scope of this manuscript.

Figure
Figure R1 a) Continuous OER photocurrent stability of CsPbBr3 photoanodes with (a) self-adhesive graphite sheet with NiFeOOH, taken from Figure 4 of Adv.Mater., b) self-adhesive glassy carbon with Ni nanopyramids and NiFeOOH, and c) with self-adhesive boron-doped diamond sheets with Ni nanopyramids and NiFeOOH.


Our previous work which showed >110 h stability (ref.31): Our previous work required the use of one dense and one porous graphite sheet and, importantly, the replacement of the porous one at 70 h with a fresh one, as shown in Figures R2a (Figure 4 of ref. 31).At 70 h, it was showing signs of deterioration, and, if the deterioration was allowed to continue, the water would have reached the perovskite in a few hours and degraded the device.Because the top graphite sheets are porous and soft, they keep swelling during the photoelectrochemical OER operation.As shown in Figure R2b (Figure S20 of ref. 31

Figure R2
Figure R2 Continuous OER photocurrent stability of CsPbBr3 photoanodes with self-adhesive graphite sheets with NiFeOOH, taken from (a) Figure 4 and (b) Figure S20 of ref. 31.

Figure R3
Figure R3 Normalized chronoamperometric measurement of FAPbBr3 photoanode at 1.23 VRHE with different filters, taken from Figure 4b of ref. 30.
caption of ref. 29.However, the Reviewer mentioned that Fehr et al. in ref.

Figure
Figure R4 (a) Schematic representation of the unassisted PEC water-splitting system using halide perovskite PECs (b) Continuous operational photocurrent of the tandem, both taken from Figure 3a and Supplementary Fig. 12 of ref. 29.
Figure: Supplementary Fig. 7 (a) Reverse current-voltage scans of a CsPbBr3 device with and without protective sheets measured as a solar cell.Performance parameters are tabulated in the inset.(b) Forward and reverse current-voltage scans of the same device with either a protective GC and BDD sheet. 1 sun illumination.Scan rate 10 mV s -1 .

Fig. 6
Fig. 6 Normalized PEC photocurrent stability test of CsPbBr3 photoanodes with various protective catalytic sheets at +1.23 VRHE under 1 sun illumination in aqueous 1 M NaOH (pH 14).The sample with GC was stopped because electrolyte turned yellow in the first hours due to degradation.The one with GS/NiFeOOH was stopped at 80 h because the device had already lost 90% of initial photocurrent.The one with GC/NiFeOOH was stopped when photocurrent reached a linear steep decay of 5% h -1 for 1 day, which was clearly irreversible.The one with BDD/NiFeOOH lost 10% photocurrent in the first hours and remained stable beyond that.The ones with GC/Ni/NiFeOOH and BDD/Ni/NiFeOOH showed practically no degradation(just 3% and 2% upon 135h, respectively).

Fig. 6
Fig. 6 Normalized PEC stability of CsPbBr3 photoanodes with various protective catalytic sheets at +1.23 VRHE under 1 sun illumination in aqueous 1 M NaOH (pH 14).The sample with GC was stopped because electrolyte turned yellow in the first hours due to degradation.The one with GS/NiFeOOH was stopped at 80 h because the device had already lost 90% of initial photocurrent.The one with GC/NiFeOOH was stopped when photocurrent reached a linear steep decay of 5% h -1 for 1 day, which was clearly irreversible.The one with BDD/NiFeOOH lost 10% photocurrent in the first hours and remained stable beyond that.The ones with GC/Ni/NiFeOOH and BDD/Ni/NiFeOOH practically showed no degradation(just 3% and 2% upon 135h, respectively).

Fig. 5
Fig. 5 ….(c) Main graph: stability test at +1.23 VRHE under 1 sun illumination.Left inset: LSV scan under 1 sun (solid line) and in dark (green dashed line) before stability test at 50 mV s -1 scan rate.Right inset: photograph of the photoanode under operation showing the evolved O2 bubbles.The electrolyte is 1 M NaOH (pH 14).
a) Reverse current-voltage scans of a CsPbBr3 device with and without protective sheets measured as a solar cell.Performance parameters are tabulated in the inset.(b) Forward and reverse current-voltage scans of the same device with either a protective GC and BDD sheet. 1 sun illumination.Scan rate 10 mV s -1 .

Fig. 4
Fig. 4 PEC performance of CsPbBr3 photoanodes with GC/Ni/NiFeOOH protective catalytic sheet.(a) LSV polarization scan (50 mV s -1 scan rate) under 1 sun illumination (solid line) and in dark (dashed green line).(b) IPCE spectrum at +1.23 VRHE.(c) OER Faradaic efficiency calculated from the experimental O2 amount and the theoretical O2 amount based on the measured photocurrent.(d) Photocurrent stability measurement at +1.23 VRHE under 1 sun illumination.Inset: SEM micrographs of Ni/NiFeOOH nanopyramid structure before and after the stability measurement and values of photocurrents at different times.Electrolyte: 1 M NaOH (pH 14).
for Fehr et al. ref. 29 results as follows: Page 5: …The energy diagram shows that electrons and holes generated in the CsPbBr3 layer would be effectively collected in the SnO2 and mesoporous carbon layers, respectively.The work function of bare GC sheet layers is 4.40 eV, and it is reduced to 4.18 eV after the electrodeposition of Ni nanopyramids…  Page 7: …To counter these undesired properties, the top GC side was roughened with silicon carbide abrasive paper and then a Ni nano-structure layer was electrodeposited, both to reduce the OER onset potential and improve the reaction surface.